This invention relates to radiant energy filtration and more specifically to a directional filter which attenuates radiant energy such as light entering the filter from outside of a pre-determined angle of incidence. In particular, the invention is useful for heads down displays in aircraft cockpits, although it may also find utility in a number of other applications using video displays under adverse lighting conditions.
Heads down displays of the type described are used to display a wide variety of aircraft navigational information in the cockpit of the craft. Often, different information is superimposed or is presented in detail which is difficult to read under varying ambient light conditions. When ambient light is low, as in night flying, it is a relatively simple task to reduce the brightness of the aircraft display. On the other hand, there are frequently ambient light conditions which require a display brightness that would be impractical either as a result of the capabilities of the display or the safety or comfort of the viewer. For example, if sunlight is creating a high glare condition, the display would not only have to overcome the glare but be bright enough for the information provided by the display to be discernible over background lighting conditions. Additionally, during the aircraft's maneuvering, lighting conditions can be expected to change rapidly. While an optical sensor can be used to sense ambient light intensity conditions, glare conditions can not always be determined by merely measuring ambient light levels.
The fixed position of the pilot-viewer enables the use of filter techniques which direct light in a single direction. For this reason, directional filters of various types have been placed in front of the CRT displays in order to block light from external sources which would tend to cause glare, while passing that light from the CRT which is traveling in the direction of the viewer. While there is a certain amount of optical amplitude (brightness) loss inherent in the use of any filter, the loss of brightness is compensated for by the decrease in glare conditions.
Prior art light filtration techniques include the use of neutral density filters. Such filters attenuate external source light as well as light from the display; however, external source light necessarily passes the filter twice and, therefore, is blocked by a square of the attenuation of light from the display itself. In the case of monochromatic displays, a notch filter is sometimes used to select the specific colors of light which are generated by or used in connection with the display. Ambient light would be highly filtered because only a small percentage of the ambient light would fall within the range of the notch filter. With the use of color display techniques, the use of a notch filter is less practical since several different wave lengths must be within the admittance bands of the notch filter.
Directional filters are used to transmit light only in a desired direction. If it is anticipated that ambient light which would cause glare would emanate from a direction other than that of the anticipated direction of the viewer from the display, it is possible to filter such ambient light using directional filters. In one type of prior art directional filter, a sheet of material is etched in order to form a large number of holes. The surfaces of the material at the holes have a high absorbency in order to eliminate reflection along the holes and at the surface of the sheet. Frequently, the sheets are stacked in order to enhance the attenuation effect of the filter. This technique is frequently expensive and may have light attenuation characteristics which are excessive.
Another directional filtration technique involves the construction of a filter plate from a plurality of sheets of thin material. The thin sheets are stacked so that each sheet is parallel to an admittance direction of light. The filter plate is taken from the stack of sheets by cutting a slice across the stack. This results in the filter plate being generally orthogonal to the direction of the individual thin sheets from which it is made, with the slice direction varying from the orthogonal direction for central viewing angles which vary from normal to the surface of the filter. This technique is subject to slight optical distortion and such a filter is expensive to produce.
It is, therefore, an object of the present invention to provide a light filter which has high attenuation ratio for unwanted-verses-wanted light. It is desired that such a filter have minimal attenuation of light in a desired viewing direction and have a maximum attenuation of light passing from beyond a given angle. It is further desired that the filter be useable with full color displays, as well as for the viewing of external conditions, as in the case of heads-up displays. It is further desired that the filter maintain a high effectiveness in adverse ambient lighting conditions with a minimum of attenuation of displayed lighting under those adverse conditions. The desired filter would be useful for direct view displays having passive and active illumination characteristics, as well as heads-up displays (HUD'S) and wind screens used for external viewing by humans and electronic sensors.